Single-crystal growth experiments in the system Na2O–K2O–CaO–SiO2 resulted in the formation of two previously unknown K–Na–Ca-silicates with the following chemical compositions as determined from electron probe micro-analysis: phase I, Na1.54(3)K0.50(3)Ca6.01(2)Si3.98(1)O15 (idealized: Na1.5K0.5Ca6Si4O15 or Na1.5K0.5Ca6[SiO4]2[Si2O7]) and phase II, Na1.41(1)K0.19(1)Ca2.20(1)Si1.993(5)O7. The low diffraction quality of the single crystals of phase II allowed only a determination of the monoclinic unit-cell parameters: a = 10.45 Å, b = 10.42 Å, c = 12.24 Å, β = 91.5° and V = 1333.8 Å3. Diffraction data collected on very small single-crystalline fragments of phase I at the X06DA beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland yielded for this compound : monoclinic symmetry, space group Pc, a = 9.0150(17) Å, b = 7.3350(10) Å, c = 11.0050(18) Å, β = 107.815(11)°, V = 692.8(2) Å3, Z = 2. Structure determination was based on direct methods. Subsequent least-squares refinements converged at R(|F|) = 0.033 for 2520 reflections and 249 parameters. Phase I is a mixed-anion silicate and contains insular [SiO4] tetrahedra as well as [Si2O7] dimers in the ratio 2:1. The cation distribution of the sodium, potassium and calcium ions among the eight different non-tetrahedral M-sites has been studied by simultaneously minimizing the differences between observed and calculated total site scattering values (as determined from single-crystal structure analysis), bond valence sums and bond lengths. Chemical composition was used as an additional constraint during the optimization process. The crystal structure can be also described as a mixed tetrahedral-octahedral network based on [SiO4] and [CaO6] polyhedra containing cavities where the remaining cations are located for charge compensation. Phase I is isostructural to β-Na2Ca6Si4O15, a high-temperature polymorph of this sodium calcium silicate. A detailed analysis of the similarities between the two compounds has been performed. Finally, a comparison between the powder diffraction patterns of different A2Ca6Si4O15 compounds is presented that may be helpful for the identification of these phases in residuals of biomass combustion.
Preparation and characterization of Na1.5K0.5Ca6Si4O15 (Na1.5K0.5Ca6[SiO4]2[Si2O7]) and Na1.41K0.19Ca2.20Si2O7: two new phases in the system Na2O–K2O–CaO–SiO2
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Volker Kahlenberg, Michael Jean-Philippe Mayerl, Daniela Schmidmair, Hannes Krüger, Martina Tribus; Preparation and characterization of Na1.5K0.5Ca6Si4O15 (Na1.5K0.5Ca6[SiO4]2[Si2O7]) and Na1.41K0.19Ca2.20Si2O7: two new phases in the system Na2O–K2O–CaO–SiO2. European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2018/0030-2767
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